Hematopoietic stem cells (HSCs) in the bone marrow are responsible for the production of mature blood cells throughout adult life. HSCs are identified by their ability to self-renew and undergo multilineage differentiation to form all major lineages of blood cells. HSCs can be transplanted into irradiated subjects, where they will engraft and give rise large numbers of diverse types of blood cells including myeloid, B, and T cells.
The state of the art for HSC purification relies on complex combinations of up to 12 markers, which target specific cell surface proteins, to isolate HSCs from bone marrow or other hematopoietic tissues by flow-cytometry. The highest enrichment of HSC activity yet reported describes combinations of markers, such those used to isolate Thy-1.1loSca-1+lineage−Mac-1−CD4−c-kit+ cells (hereafter referred to as Mac-1−CD4−c-kit+), from which one out of every five intravenously injected cells are able to home to bone marrow and engraft (see, e.g., Uchida et al., J Exp Med. 1992 Jan. 1; 175(1):175-84; Morrison et al., Immunity. 1994 November; 1(8):661-73; and Morrison et al., Development. 1997 May; 124(10):1929-39, all of which are herein incorporated by reference). HSCs are characterized by the ability to undergo long-term multilineage reconstitution (for extended periods of time and usually for the life of a subject, in the case of a murine subject, for more than 16 weeks to life), whereas other populations of hematopoietic progenitors only transiently form mature blood cells. For example, HSCs give rise to non-self-renewing multipotent progenitors (MPPs) that can be isolated as Thy-1.1loSca-1+Mac-1loCD4lo bone marrow cells (See, Morrison et al., 1994 and Morrison et al. 1997 supra). These cells give rise to myeloid B and T cells for less than 6 weeks after injection into irradiated mice, for example.
Highly purified HSCs are increasingly being used clinically, such as for autologous transplants into patients after high-dose chemotherapy. In this setting it is critical to isolate HSCs with the maximum degree of purity, to minimize contamination by immune effector cells (such as lymphocytes) or cancer cells. Since few markers or cell surface proteins have been identified that are highly specific to HSCs, it is not possible to identify or collect these cells based on simple combinations of one or two markers. As a result, it has only been possible to highly purify HSCs using complex combinations of many markers. These complex combinations of markers have not been practical to use clinically. As a result, simplified combinations of one or two markers (e.g., monoclonal antibodies that can identify CD34+ or CD34+CD38− cells) have often been used for the clinical isolation of HSCs, but these yield much lower levels of purity. Thus it would be desirable to identify new cell surface proteins, and/or markers corresponding thereto, that improve and simplify the purification of HSCs to facilitate their clinical use.